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Drag coefficient distribution on a wing at 90 degrees to the wind [ECN-C--95-061]
Gepubliceerd door: Publicatie datum:
ECN 1996
ECN publicatienummer: Publicatie type:
ECN-C--95-061 ECN rapport
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The drag coefficient along a horizontal axis wind turbine blade or anairplane wing, for about 90 degrees angle of attack, is the studied quantity (CD90) in this report. The arguments to assess CD90 are (1) that a typical design feature for the rotor is to have blades which are unpitchable, the method to avoid too high loading being handled by natural stall (stall control). Ultimate loadings for such turbines may come from the situation when a storm wind, parallel to the main shaft, hits the parked rotor. Flat wise bending occurs, creating potentially very high stress levels along the blade; (2) a similar need as in 1 may arise for a pitched rotor for which there is always a combination of angles for pitch, azimuth and yaw for which the blade presents its flat area to the wind; (3) the need to assess the coefficients of both lift (CL) and drag (CD) for high angles of attack while the turbine is operating. The connection between CD90 and CL and CD, at arbitrary angles of attack, is briefly described in this report; and (4) some rotors have aerodynamic brakes in the form of pivoting tips. Their braking capability is calculated for very high angles of attack typically around 90 degrees. In order to find a method for the CD90 assessment two different hypotheses are investigated. These hypotheses are basically concerned with the distribution of CD90 along the blade. This distribution can be constant (Method 1) or variable (Method 2). The basis for finding CD90 comes from literature covering statistics from many examples of flat plates without taper and four sets of data from one small and three full scale wind turbine blades. Following Method 1 an approximate value for CD90 of 1.4 constant along the blade is used. It is good for the majority of today's wind turbine blades. However, for more slender blades the number is higher. It is concluded that the amount of data from real blades is too small for an ascertain assessment of the quality of Method 2. Yet, Method 2 should be useful whenever alternative designs are compared. Finally, a recommended path of calculation is presented in the form of a stepwise algorithm. 20 figs., 3 tabs., 14 refs.

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